The general feel on the forums and around the intertubes in general is this:

If you have WiiConnect24 on, then Nintendo may be able to do a drive-by update. You'll turn your Wii on one day to find it updated itself overnight and now your homebrews don't work.

It's hard to make a block for this for a few reasons:

1) Nintendo can change up their game at any time. When they do, you're vulnerable again until an update to the update blocker becomes available.

2) There is no telling how Nintendo's servers interpret a failed update over WiiConnect24. It may read it as an unspecified error or it may read it as you intentionally blocking it, meaning your console is modified, meaning when they get the opening, they'll shut you down. Again, Nintendo and independent Wii homebrew programmers would be in an arms race here.

It's easiest and safest just to turn off the 24/7 connection and take advantage of the fact that you wi-fi for gaming without issue (usually) with homebrew installed.

You can still play games online safely but if you use cheat codes you can get banned. Also some channels need wiiconnect24 on to use. Some such as the internet channel just need an internet connection but do not require wiiconnect24.